Mercury is a naturally occurring element that is known to have a toxic effect on human beings and animals in very low concentrations. Mercury's high toxicity combined with its penchant for bioaccumulation make it of particular concern among heavy metals. In particular, mercury can affect the nervous system, with fetuses, infants, and children being particularly sensitive to the effects of mercury. Methyl mercury is a highly toxic form of mercury found in sediments and water which is taken up by small organisms as they feed and subsequently accumulates in fish and shellfish that feed on such organisms. People and fish-eating wildlife, in turn, become exposed to methyl mercury when they ingest fish and shellfish containing methyl mercury. Detection and accurate measurement of methyl mercury in environmental and biological samples is thus of great interest. There are several ways of determining total mercury in environmental samples. However, speciation of mercury is more difficult.
Most methods currently employed in the analysis of methyl mercury levels are based on “Method 1630,” entitled Methyl Mercury in Water by Distillation, Aqueous Ethylation, Purge and Trap, published by the U.S. Environmental Protection Agency. While this method is designed for monitoring water quality, it has been adapted for use with other types of samples by modifying the preparation steps. A sample is generally first subjected to either digestion, wherein the methyl mercury is leached from the sample; distillation, wherein the methyl mercury is carried from a distillation vessel and condensed in a receiving vessel, leaving many compounds other than water behind; or solvent extraction, with the resulting solvent containing the methyl mercury that is used for subsequent analysis.
The treated sample is then added to a vessel containing a larger volume of high purity de-ionized water, a buffer, and an ethylating reagent (generally sodium tetraethylborate). The ethylating reagent combines with various forms of mercury present in the sample resulting in formation of more complex “ethylated” molecules. The ethylated forms of mercury are fairly volatile and can therefore be stripped from the solution by bubbling a gas through the liquid. A gas, such as nitrogen, is utilized to purge the solution and carry the vapor to a tube that has been packed with a material that will retain a wide variety of substances and thus acts as a trap, retaining the ethylated forms of mercury. The trap packing material is designed such that the trapped molecules can be released by thermal desorption (i.e., heating the packing material and trapped molecules to the point where molecular vibrations overcome the attraction forces that keep the two together).
The trap is removed from the purge vessel and dried by allowing dry nitrogen gas to pass through it for a short time. After drying, the trap is manually connected to an inert gas source (generally argon or helium) and heated (for example using a nichrome wire coil) to release the ethylated species which are then carried out of the trap by the inert carrier gas flow to a gas chromatography (GC) column held at a fixed temperature. The different species, or forms, of mercury exit this column at different times based on their molecular mass, the temperature of the column and the gas-flow rate, with the smaller mercury species exiting the GC column before the larger species.
As the gas exits the GC column, it carries the different time-resolved mercury species into a quartz tube packed with quartz wool which is held at a very high temperature (referred to as a “pyrolytic” column) where, regardless of their molecular form, the mercury species are decomposed so that the atomic mercury is no longer bonded within a molecule. The resulting atomic mercury vapor is detected for each mercury species using, for example, a cold vapor atomic fluorescence spectrophotometer (CVAFS) such as that described in U.S. Pat. No. 5,731,873. The amount of each mercury species can then be quantified by comparison with results obtained for standard samples containing known levels of mercury.
This manual method for analyzing methyl mercury levels is time consuming and requires significant operator input. As with all manual techniques, there is significant inherent variability, with the method being prone to operator error. There thus remains a need in the art for systems and methods for the detection of low levels of contaminants, such as methyl mercury, which are both cost- and time-efficient, and require minimum operator input.